The development of spontaneous hypertension will be studied in a strain of rats which is highly prone to show severe hypertension with hypertensive pathological lesions. By using a combination of fluorescent tracers, intravital microscopy and electron microscopy, the development and progression of mesenteric arterial and arteriolar abnormal endothelial permeability, sustained smooth muscle contraction and relaxation, intimal and medial hyperplasia, fibrinoid deposition, necrosis and fibroplasia will be studied. The effect of vasoactive agents on the abnormally contracted and/or dilated arteries and arterioles will be tested, applied locally by microiontophoresis. A continuous recording of the blood vessels on video tape is made during the testing of vascular permeability and smooth muscle responsiveness, as well as during the fixation and embedding procedures. The data thus recorded can be played back instantly at any time for evaluation, measurement, identification, and control of the precise initial sites, within the vascular tree, of abnormal arterial and arteriolar reactivity.